The present invention relates to a production process of a toner for development of electrostatic images for developing electrostatic latent images formed by an electrophotographic process, electrostatic recording process or the like, and more particularly to a process for producing a polymerized toner for development of electrostatic images, which is markedly reduced in the content of a residual polymerizable monomer, prevented from emitting offensive odor upon fixing of the toner and excellent in environmental safety.
In an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus, the formation of an image is conducted through (1) an exposure step of conducting exposure to a light pattern of the image on a photosensitive member uniformly and evenly charged to form an electrostatic latent image (electrostatic image), (2) a development step of applying a toner to the electrostatic latent image to form a toner image (visible image), (3) a transfer step of transferring the toner image to a transfer medium such as paper or OHP film, and (4) a fixing step of fusion-bonding the toner image to the transfer medium by heating and pressing, or the like.
In order to develop the electrostatic latent image formed on the photosensitive member, a developer composed mainly of a toner for development of telectrostatic images (hereinafter referred to as xe2x80x9ctonerxe2x80x9d merely) is used. As the toner, are used colored resin particles containing at least a binder resin and a colorant, and additionally containing various kinds of additives such as a charge control agent as needed.
Processes for producing a toner are roughly divided into a grinding process and a polymerization process. The grinding process is a process of producing colored resin particles by melting a binder resin synthesized by polymerization in advance and kneading it with additive components such as a colorant, a charge control agent and a parting agent, and then grinding and classifying the kneaded product. The colored resin particles are generally called xe2x80x9cground tonerxe2x80x9d.
The polymerization process is a process of producing colored polymer particles (i.e., colored resin particles) by mixing a polymerizable monomer with additive components such as a colorant, a charge control agent and a parting agent to prepare a polymerizable monomer composition and then polymerizing the polymerizable monomer composition by suspension polymerization, emulsion polymerization, dispersion polymerization or the like. The colored polymer particles are generally called xe2x80x9cpolymerized tonerxe2x80x9d. In the polymerization process, the polymer component formed by the polymerization becomes a binder resin to directly form the colored polymer particles. Incidentally, the colored polymer particles obtained by the polymerization process may be referred to as xe2x80x9cpolymer particlesxe2x80x9d merely in some cases in the present specification.
In any technique of the grinding process and the polymerization process, it is difficult to completely react the polymerizable monomer in the polymerization step for forming the binder resin. Therefore, an unreacted polymerizable monomer remains. As a result, such a monomer comes to be contained in the toner. When the toner containing the polymerizable monomer is used in an image forming apparatus, the polymerizable monomer is vaporized out of the toner by heating in a fixing step, or the like to worsen a working environment or emit offensive odor. When the content of the polymerizable monomer in the toner is high, the toner tends to undergo blocking during its storage to aggregate or to cause an offset phenomenon or toner filming on individual members in the image forming apparatus.
In the grinding process, a polymerizable monomer is polymerized in advance to synthesize a binder resin. Therefore, an unreacted polymerizable monomer can be removed with relative ease by a washing treatment or heat treatment after the polymerization. Thereafter, the binder resin is melted and kneaded with various kinds of additive components, and the resulting mixture is ground and classified, whereby a ground toner low in content of the residual polymerizable monomer can be provided. On the other hand, in the polymerization process, a polymerized toner is obtained as polymer particles containing the various kinds of additive components by polymerizing the polymerizable monomer. Therefore, it is difficult to remove the residual unreacted polymerizable monomer.
The residual polymerizable monomer in the polymerized toner is easy to be adsorbed on the various kinds of additive components, so that its removal is difficult compared with the case of the binder resin alone. Even when the polymerized toner is fully washed after the polymerization, it is difficult to remove the residual polymerizable monomer adsorbed within the polymerized toner. The attempt to remove the residual polymerizable monomer by the heat treatment of the polymerized toner results in aggregation of the polymerized toner.
In recent years, there has been a demand for development of toners capable of being fixed at a temperature lower than before for the purpose of achieving the speeding-up of copying or printing and energy saving. There has also been a demand for development of color toners for color output, which are low in melt viscosity and easy to be uniformly melted upon fixing. As described above, there is an increasing demand in market for development of toners (hereinafter referred to as xe2x80x9clow-temperature fixing tonerxe2x80x9d) capable of being fixed at a temperature lower than before. When the glass transition temperature or melt viscosity of a binder resin component is lowered for meeting such requirements, however, it is more and more difficult to reduce the content of the residual polymerizable monomer while preventing aggregation of the resulting polymerized toner.
Many of polymerized toners are produced by a process of suspension-polymerizing a polymerizable monomer composition containing a polymerizable monomer and various kinds of additive components in an aqueous medium. In the production steps of a toner by the polymerization process, general post-treatment steps after a polymerization step include washing, dewatering and drying steps. Many processes of conducting a removing treatment of an unreacted polymerizable monomer in a post-treatment step after polymerization for the purpose of reducing the content of the unreacted polymerizable monomer remaining in the resulting polymerized toner have heretofore been investigated.
Specifically, for example, (1) a process of treating a polymerized toner after a drying step, (2) a process of treating a polymerized toner after a dewatering step, and (3) a process of treating a suspension containing a polymerized toner formed by suspension polymerization are known.
As the process (1) of treating the polymerized toner after the drying step, there is known a process in which a dry polymerized toner is heated and subjected to a deaerating treatment under reduced pressure (Japanese Patent Application Laid-Open No. 92736/1995). However, this process is difficult to reduce the content of the polymerizable monomer remaining in the polymerized toner to an extent lower than 150 ppm. In addition, the heat treatment of the dry polymerized toner shows a tendency for polymerized toner particles to aggregate to one another. In the case of low-temperature fixing toners suitable for use in high-speed printing or as color toners, the tendency to aggregate upon the heat treatment becomes particularly strong. The reason for it is that those having a low glass transition temperature, melting point or softening point are used as a polymer component, which will become a binder resin, and various kinds of additive components in the low-temperature fixing toner for the purpose of lowering the fixing temperature thereof. When the polymerized toner is heat-treated in the post-treatment step after the polymerization, these components are softened to become liable to aggregate. Accordingly, the above-described process is particularly unpractical for low-temperature fixing toners.
As the process (2) of treating the polymerized toner after the dewatering step, there has been proposed a process in which a polymerized toner in a wet state is dried under reduced pressure while pouring a gas into the toner (Japanese Patent Application Laid-Open No. 207122/1998). However, this process is difficult to reduce the content of the polymerizable monomer remaining in the polymerized toner to an extent lower than 100 ppm. Further, in this process, the polymerized toner particles are placed in a vacuum dryer to dry the toner particles while stirring them under heat. Therefore, when the process is applied to a polymerized toner suitable for low-temperature fixing, the polymerized toner particles may possibly aggregate to one another.
As the process (3) of treating the suspension containing the polymerized toner, there has been proposed a process in which a medium of the suspension is distilled off while blowing the saturated vapor of the medium of the suspension into the suspension in the latter half of the suspension polymerization reaction or after completion of the reaction (Japanese Patent Application Laid-Open No. 100485/1993). The publication states that a residual polymerizable monomer can be reduced to 70 ppm according to this process (Example 3). However, this process makes the polymerized toner particles easy to aggregate to one another by shearing force upon the contact of the saturated vapor of the medium of the suspension with the polymerized toner particles and shows a tendency to cause scale due to adhesion of the polymerized toner particles to the inner wall of an apparatus and form great agglomerate of the polymerized toner particles. Therefore, this process does not always provide a polymerized toner high in flowability. In particular, the polymerized toner particles have a stronger tendency to aggregate to deteriorate flowability as the particle diameter of the polymerized toner particles becomes smaller. The aggregation of the polymerized toner particles impedes the flowability of the polymerized toner particles to deteriorate the quality of an image formed with such a toner.
As the process (3) of treating the suspension containing the polymerized toner, besides the above, there has been proposed a process in which an aqueous medium of the suspension is distilled off while adding the aqueous medium to the suspension after completion of the polymerization reaction (Japanese Patent Application Laid-Open No. 66613/1993). However, this publication does not disclose the degree of reduction of the content of the residual polymerizable monomer in the polymerized toner. The polymerizable monomer used in the suspension polymerization slightly dissolves in water. Therefore, a residual polymerizable monomer contained in the resulting polymerized toner can be transferred to an aqueous medium if the polymerizable monomer can be brought into contact with a great amount of the aqueous medium. However, the amount of the polymerizable monomer transferred to the aqueous medium from the polymerized toner particles is lessened if the amount of the aqueous medium, with which the polymerizable monomer is brought into contact, is small.
However, according to the process of distilling off the aqueous medium while gradually adding the aqueous medium to the suspension as proposed in this publication, the efficiency to dissolve out the unreacted polymerizable monomer contained in the polymerized toner particles is not very high. Accordingly, even if this process is applied to the mass production of a polymerized toner, it cannot be expected that the content of the residual polymerizable monomer is reduced.
In this publication, it is proposed as a process of distilling off the aqueous medium to distill off the aqueous medium in an amount of 5 to 100% by weight of the suspension while heating the suspension to at least a temperature corresponding to the peak top of an endothermic peak of the resulting polymerized toner by differential scanning calorimetry (DSC). Specifically, in Examples of this publication, water is distilled off while heating the suspension to 150xc2x0 C. under reduced pressure to add water. Since the temperature conditions far exceeds the boiling point of water, the aqueous medium added is easy to be distilled off. However, the distilling-off at such a high temperature is severe as treatment conditions for the polymerized toner and thus liable to incur the adhesion of the polymerized toner to the inner wall of the apparatus or coagulation thereof. In particular, this tendency more and more increases in polymerized toners which contain low-softening substances and the binder resin itself of which has a low glass transition temperature, such as the low-temperature fixing toners.
As described above, the means for reducing the content of the residual polymerizable monomer by the post-treatment steps after polymerization in the production process of a polymerized toner are variously investigated. However, such means are yet insufficient. In particular, it is thus difficult to meet the severe requirement level for environmental safety in recent years that the content of the residual polymerizable monomer is lower than 100 ppm. Further, there has not been yet found any process for reducing the content of the residual polymerizable monomer, which is suitable for industrial production of low-temperature fixing toners.
It is an object of the present invention to provide a production process of a polymerized toner, by which a polymerized toner extremely low in the content of a residual polymerizable monomer can be produced with high productivity and without impeding the excellent properties of the resulting polymerized toner.
More specifically, the object of the present invention is to provide a production process of a polymerized toner, by which a polymerizable monomer remaining in polymer particles (polymerized toner) formed can be removed with good efficiency in a post-treatment step after polymerization while preventing bubbling of an aqueous dispersion containing the polymer particles and aggregation of the polymer particles.
The present inventors have carried out an extensive investigation with a view toward achieving the above object. As a result, it has been found that when an aqueous dispersion containing polymer particles formed is subjected to stripping under reduced pressure in an evaporator after completion of a polymerization reaction, and at this time the aqueous dispersion within the evaporator is led to an external heat exchanger to heat it, and the heated aqueous dispersion is subjected to stripping under reduced pressure while circulating the heated aqueous dispersion to a liquid phase within the evaporator, polymer particles (polymerized toner particles) markedly reduced in the content of a residual polymerizable monomer can be provided while preventing the aggregation of the polymer particles and the occurrence of scale. It has also been found that when the quantity of heat supplied is increased step by step or continuously upon the stripping of the aqueous dispersion under reduced pressure, the efficiency to reduce the content of the residual polymerizable monomer can be enhanced while preventing the aggregation of the polymer particles and the occurrence of scale. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a process for producing a polymerized toner, comprising the step of polymerizing a polymerizable monomer composition containing at least a polymerizable monomer and a colorant in an aqueous medium, wherein an aqueous dispersion containing polymer particles formed is subjected to stripping under reduced pressure in an evaporator after polymerization, and at this time the aqueous dispersion within the evaporator is continuously taken out of the evaporator and heated in an external heat exchanger, and the heated aqueous dispersion is returned to a liquid phase within the evaporator and subjected to stripping under reduced pressure within the evaporator while circulating the aqueous dispersion, thereby distilling off volatile organic compounds.
According to the present invention, there is also provided a process for producing a polymerized toner, comprising the step of polymerizing a polymerizable monomer composition containing at least a polymerizable monomer and a colorant in an aqueous medium, wherein an aqueous dispersion containing polymer particles formed is subjected to stripping under reduced pressure after polymerization, and at this time the quantity of heat supplied to the aqueous dispersion is increased step by step or continuously to subject the aqueous dispersion to stripping under reduced pressure, thereby distilling off volatile organic compounds.